Cenospheres are hollow spherical-form particles which are contained in fly ash. They have a white to light grey colour, are composed predominantly of silica (59%), alumina (28%) and iron oxide (5%), contain mainly nitrogen within the sphere, have a size from 45 to 150 micrometers, a specific gravity of 0.55 and a dry density (less than 0.5% moisture) of 330 Kg/m.sup.3. The cenospheres have application as aggregate for light-weight concrete and are suitable for use in refractories, plastics materials, paints, sound insulation material, general filler applications and thermal insulating materials.
The cenospheres are separated from the other constituents of fly ash by water flotation processes but subsequent drying of agglomerated cenosphere material has been found to be extremely difficult, using conventional drying apparatus, due to the low density of the material.